Since it was introduced in 1975, ion chromatography has become a widely used analytical technique for the determination of anionic and cationic analytes in various sample matrices. In ion chromatography, dilute solutions of acids, bases, or salts are commonly used as the electrolytes in chromatographic eluents.
Traditionally, these eluents are prepared off-line by dilution with reagent-grade chemicals. Off-line preparation of chromatographic eluents can be tedious and prone to operator errors, and often introduces contaminants. For example, dilute NaOH solutions, widely used as the electrolytes in eluents in the ion chromatographic separation of anions, are easily contaminated by carbonate. The preparation of carbonate-free NaOH eluents is difficult because carbonate can be introduced as an impurity from the reagents or by adsorption of carbon dioxide from air. The presence of carbonate in NaOH eluents often compromises the performance of an ion chromatographic method, and can cause an undesirable chromatographic baseline drift during the hydroxide gradient and even irreproducible retention times of target analytes. Therefore, there is a general need for convenient sources of high purity acid, base, or salt for use as eluents in the ion chromatographic separations.
The continuous operation of an ion chromatography system can consume a significant amount of eluents. The consistent preparation of such large amount of the eluent as well as the disposal of the used eluent can pose serious logistical challenges to the system operators in terms of costs and labor, especially in cases where unattended or less frequently attended operations are required. Even though it overcomes a number of issues associated conventional approaches of eluent preparation in ion chromatography, the use of on-line electrolytic eluent generation devices still requires a constant supply of high purity water from an external source for continuous operation and waste disposal issue remains.
U.S. Pat. No. 7,329,346 describes ion chromatography systems capable of recycling eluents. In one embodiment, the electrolytic suppressor is operated in the recycle mode. The net result of the electrochemical processes in an electrolytic suppressor is that the combined effluent from the suppressor anode and cathode chambers is a mixture of hydrogen gas, oxygen gas, and the aqueous solution containing the eluent components, the ions from the sample injected, and possibly some trace components derived from the operations of the separation column and suppressor. The effluent from the outlet of the electrolytic suppressor regenerant chamber is passed through the catalytic gas elimination column packed with a Pt catalyst that induces the reaction between hydrogen gas and oxygen gas to form water. The catalytic gas elimination column serves several important functions. First, it provides an elegant means to conveniently eliminate the build up of hydrogen and oxygen gases and thus facilitates the operation of continuous eluent recycle. Second, the water-forming reaction of hydrogen and oxygen is stoichiometric in the column, and the amount of water formed is expected to be in principle the same as the amount of water consumed originally to produce hydrogen and oxygen gases in the electrolytic operation of the suppressor. In the above embodiment, an analyte trap column is placed after the outlet of a conductivity detector to trap analyte ions. Additionally, ion exchange eluent purification columns packed with appropriate ion exchange resins are used to further purify the regenerated eluent for use for re-use as the ion chromatographic eluent in the separation process.